Burial conditions affect light responses of Datura ferox seeds

The dormancy pattern of buried seeds of Datura ferox was studied by testing germination responses to red light (R), far-red light (FR) or darkness after exhumation at regular time intervals. The seeds were deeply dormant at the time of burial in winter. After three months (spring), exhumed seeds began to respond to an R pulse. Summer temperatures did not induce secondary dormancy. During winter of the second year, seed germination after an R pulse increased further; however, germination after an FR pulse or in darkness also increased and reached values close to R. Changes in seed germination after an R or an FR pulse and in darkness were studied in a factorial design experiment, combining different depths of burial (0.5, 5 and 10 cm), presence of vegetation (with or without vegetation canopies), and date of exhumation (4 or 7 months after burial). The seeds placed at 0.5 cm depth did not germinate in any light treatment. A very high sensitivity to light, indicated by the promotion of germination by an FR pulse compared to darkness, was exhibited only in seeds buried at 5 cm in a soil without vegetation. A typical R/FR reversible response was observed in seeds exhumed at 5 and 10 cm in almost all situations of burial; except in a soil without vegetation, at 10 cm depth, in late summer when even dark controls showed high germination. It is concluded that buried seeds of D. ferox are released from dormancy in two steps, the first step during the spring of the first year and the second step during the subsequent winter. This process is not obviously controlled by summer temperatures. The acquisition of a very high sensitivity to light depends on vegetation cover and depth of burial, and thus it is predicted to be affected by agricultural practices.